Jökull - 01.12.1974, Page 19

Fig. 13. a) A schematic
picture of the distribu-
tion of pj and P in the
Grímsvötn region (see the
defination of p^ and P in
the text). The closed area
represents the subglacial
lake. The lake is sur-
rounded by a barrier.
The width of the barrier
is smallest at the south-
east side of the lake. The
reason for this is that the
glacier surface lies above
the Grimsvötn level
north of the lake but be-
low the Grímsvötn level
south of the lake, see
Figs. 6 and 14.
b) A map showing the
evidence of Fig. 13 a.
The map gives the distri-
bution of Pj and P
around Grímsvötn when
the water level in the
lake has risen 100 m (up
to the 1430 m elevation).
The values for p4 and P
are derived frorn the map
of the average elevation
of the glacier bed (Fig. 7).
According to the average values for p4 and P, there is still a narrow seal sout-east of the lake.
ít is suggested that the route of the jökulhlaup lies close to the line ABCDEF.
Mynd 13. Kort af Grímsvatnasvœðinu. Kortið sýnir meðalgildi fyrir bceði farg jökulsins (pþ) og
hugsanlegan vatnsprýsting (P) við botn jökulsins, þegar hæst er i Grímsvötnum. Samkvœmt þessari
mynd ættu vötnin enn að vera lokuð. Myndin sýnir hins vegar ekki frávik i pt og P, sem stafa
af skorningum i botni jökulsins austur af Grimsvötnum. Einnig má vcenta þess, að sveiflur verði
í pt við hreyfingu jökulsins, t. d. árssveiflur. Sveiflur og frávik frá meðalgildum á kortinu gcetu
skýrt, hvers vegna hlaup hefst, þegar vatnsborð Grimsvatna hefur risið um 100 m.
area which subsided during the exceptional
jökulhlaup in 1938 (Fig. 9 and p. 22) and
would include the cauldrons immediately to the
west of the present water divide. These cauld-
rons have drained water by jökulhlaups to the
river Skaftá (Fig. 1) since 1955 but previously
this water was presumably drained to Gríms-
vötn (Björnsson, in press).
No accurate measurements have been carried
out on the heat flux of any high-thermal area
in Iceland but Bödvarsson (1961) estimates
Q = 6.5 • 1010 J/yr for the Torfajökull area
(Ag = 100 km2), which gives a heat flux density
iþ = 20 W/m2. The present estimate of Q =
1.5 • 1017 J/yr is about twice the total energy
flux estimated for the Torfajökull area, which
so far has been considered the strongest geo-
thermal area in Iceland. If one accepts both esti-
mates the conclusion would be that the Gríms-
vötn geothermal area is of the same extent as
JÖKULL 24. ÁR 1 7